Ink jet recording system in which aqueous ink is ejected through a nozzle having fine pores to form an image on recording paper is widely used in terminal printers, facsimiles, plotters, sheet feeding printers, etc., due to low noise during recording, ease of performing color recording, possibility of performing high-speed recording, lower cost than other printing devices, and so forth.
Recently, demand has increased for high performance of printing paper which is used in an ink jet recording system due to increasing wide spread use of printers and development thereof to enhance high definition and high-speed performance as well as appearance for digital cameras in the field.
Accordingly, ink jet printing paper having excellent recording properties including a high ink absorptivity, a high recording density, a high water resistance, and, in particular, a high image quality and surface glossiness equivalent to a silver halide photograph is strongly awaited.
As a method for imparting glossiness to ink jet printing paper, a method in which the surface of a coating layer is smoothened by being passed between rollers to which pressure or temperature is applied using a device, such as a super calender, (i.e., calender finish) is generally known.
However, the glossiness of printing paper obtained by the calender finishing is not sufficient. In addition, there are problems in that the ink absorptivity thereof is reduced due to reduction of voids in a coating layer, and hence, feathering tends to be easily caused.
Other than the above-mentioned calender finishing, a number of methods have been proposed which improve the glossiness of paper by providing an ink receiving layer formed by an ink absorbing resin, such as starch, gelatin, water-soluble cellulose resin, polyvinyl alcohol, polyvinyl pyrrolidone, denatured polyurethane, on a luster surface of a smooth plastic film or a resin coated paper.
However, although the printing paper obtained by the above methods have some degree of glossiness, the ink absorptivity thereof is not sufficient and ink drying speed is slow. Accordingly, it is not easy to handle the printing paper and there are problems that uneven ink absorption tends to be caused and that water resistance is low and curl is caused.
On the other hand, a method in which a coating layer is provided which includes ultra fine pigment powder of colloidal silica having small particle size is proposed in, for example, Japanese Laid-open Patent Application No. Hei. 2-274587, Japanese Laid-open Patent Application No. Hei. 8-67064, Japanese Laid-open Patent Application No. Hei. 8-118790, Japanese Laid-open Patent Application No. 2000-37944, and Japanese Laid-open Patent Application No. 2001-353957.
However, although the printing paper obtained by the above method has a glossiness of some degree, voids are not sufficiently formed in the coating layer, and a satisfactory ink absorptivity cannot be obtained.
Ink jet printing sheet having at least a layer including synthetic silica having an average particle size of primary particle of 50 nm or less formed by a gas phase method and a layer including a colloidal silica in that order viewed from a supporting sheet is proposed in Japanese Laid-open Patent Application No. 2000-37944. However, although the glossiness and anti-abrasion property thereof can be improved, its void rate is reduced by the primary powder, such as colloidal silica, and hence, the ink absorptivity thereof tends to be lowered. Also, if the coating amount is reduced in order to maintain the ink absorptivity, interference patterns are generated, lowering the quality of the luster surface, and a satisfactory glossiness cannot be obtained.
Also, ink jet printing sheet having at least a layer including synthetic silica having an average particle size of primary particle of 30 nm or less formed by a gas phase method and a layer including cationic colloid particles in that order viewed from a supporting sheet is proposed in Japanese Laid-open Patent Application No. 2001-353957. However, although feathering of image after printing and water resistance may be improved, there is a problem that the ink absorptivity thereof is lowered as in Japanese Laid-open Patent Application No. 2000-37944.
As described above, a method for improving glossiness without significantly decreasing ink absorbing speed is not disclosed in any of Japanese Laid-open Patent Application No. Hei. 2-274587, Japanese Laid-open Patent Application No. Hei. 8-67064, Japanese Laid-open Patent Application No. Hei. 8-118790, Japanese Laid-open Patent Application No. 2000-37944, Japanese Laid-open Patent Application No. 2001-353957, and so forth.
As a method for imparting glossiness other than the methods described above, so called cast coating methods are known in which a wet coating layer is pressed against a heated calender roll having a specular surface and dried to copy the specular surface (for example, refer to U.S. Pat. No. 5,275,846 and Japanese Laid-open Patent Application No. Hei 7-89220).
Cast coating methods which are generally known are as follows:
(1) a wet cast coating method in which pigment composition having pigment and adhesive as main components is coated on a base paper, and while a coating layer is in a wet state, it is pressure welded against a specular-finished heated calender roll and dried to obtain a glossy finish;
(2) a gel cast coating method in which a wet state coating layer is gelatinized by acid, salt, heat, and so forth, and is pressure welded against a heated calender roll and dried to obtain a glossy finish; and
(3) a rewet cast coating method in which a coating layer of wet state is once dried and then wet plasticized using a rewetting solution, and is pressure welded to a heated calender roll and dried to obtain a glossy finish.
Although each of the above wet casting methods are recognized as a distinguished techniques among skilled persons in the field, the methods are similar in terms of pressure welding a surface of a coating layer in a wet plasticizing state against a calender roll, drying, and separating from a heated calender roll to copy a specular surface.
Cast coating paper obtained by the above wet casting method is mainly used for high quality printing matter since it has a high surface glossiness and a high surface smoothness and excellent printing effect can be obtained as compared with normal printing paper which is calender finished.
However, there are various problems for applying the cast coating paper to ink jet printing paper. For example, the above-mentioned cast coating paper acquires high glossiness due to the presence of a film-forming material, such as an adhesive, contained in the pigment composition forming a coating layer, which copies the surface of a calender roll of a cast coater as disclosed in U.S. Pat. No. 5,275,846. The porosity of the coating layer is lost by the presence of the film-forming material, and absorptivity of ink during ink jet recording is significantly reduced. For that reason, it is important to make the coating layer porous so that it easily absorbs ink and improves the ink absorptivity of the cast coating paper. On the other hand, in order to obtain a high quality image equivalent to a silver halide photograph, it is necessary that a uniform film be formed in the coating layer so that ink which is ejected through fine nozzles of an ink jet printer can be reproduced on the paper without cracks. However, it is extremely difficult to realize both a uniform film having no cracks and a porosity using a conventional wet cast coating method.
Also, in the cast coating paper, it is necessary that water components in the coating material be vaporized through the backside because the wet coating material is contacted the calender roll and dried. For this reason, vapor will remain in the coating layer if a supporting sheet having a significantly low permeability, such as resin coated paper or film, is used. Since the volume of vapor is much larger than the volume of water, the captured vapor will raise the supporting sheet. At that time, the weakest portion of the coating layer would be damaged.
For example, if the bonding of the coating material to the specular-finished heated calender roll is weak, peeling occurs at the boundary between the coating layer and the calender roll, and a so-called insufficient adherence that causes insufficient copying of the specular surface of the calender roll is caused. On the other hand, if the strength of an undried coating layer is weaker than the bonding strength between the calender roll and the coating material, the coating layer will be broken inside thereof and a part of the coating material will attach on the surface of the calender roll to stain the calender roll. In either case, a clear casting surface cannot be formed and becomes a cause of problems in terms of quality and operation.
Accordingly, a great difficulty is associated with obtaining a cast coating paper using a low permeability or nonpermeable supporting sheet, such as a resin coated paper and film.
Also, it is known that a defect called cockling may be observed during ink jet printing in which printing paper is elongated and wrinkled due to the effect of solvent, such as water, contained in ink. Cockling not only disturbs the appearance of printing matter but also makes the printing paper contact a recording head to stain the printing paper. This may lead to breakage of the printing paper or malfunction of the recording head.
In order to prevent the generation of cockling, it is effective to use a supporting sheet which is not enlarged by the solvent of ink, or to provide a layer between an ink receiving layer and a supporting sheet, which does not permeate the solvent of ink. For example, if a low permeability or nonpermeable supporting sheet, such as resin coated paper and film, is used, cockling can be effectively prevented.
That is, an object of the present invention is to provide ink jet printing paper including a low permeability or nonpermeable supporting sheet, having a high surface glossiness and a high dot reproducibility equivalent to a silver halide photograph level, the ink jet printing paper having excellent ink absorptivity and ink absorbing rate, and a high recording density, and which is capable of preventing cockling by ink solvent.